The present invention relates, in general, to circulating fluidized bed (CFB) boiler arrangements and, more particularly, to a CFB boiler arrangement having a selective non-catalytic reduction (SNCR) system employed downstream of the CFB boiler furnace to achieve enhanced NOx reduction capability.
CFB boiler arrangements are known and used in the production of steam for industrial processes and/or electric power generation. See, for example, U.S. Pat. Nos. 5,799,593, 4,992,085, and 4,891,052 to Belin et al.; U.S. Pat. No. 5,809,940 to James et al.; U.S. Pat. Nos. 5,378,253 and 5,435,820 to Daum et al.; and U.S. Pat. No. 5,343,830 to Alexander et al. In a CFB boiler furnace, reacting and non-reacting solids are entrained within the furnace enclosure by the upward gas flow that carries solids to the exit at the upper portion of the furnace, where the solids are separated by impact type particle separators. The impact type particle separators are placed in staggered arrays to present a path which may be navigated by the gas stream, but not the entrained particles. The collected solids are returned to the bottom of the furnace. One CFB boiler arrangement uses a plurality of impact type particle separators (or concave impingement members or U-beams) at the furnace exit to separate particles from the flue gas. While these separators can have a variety of configurations, they are commonly referred to as U-beams because they most often have a U-shaped configuration in cross-section.
Impact type particle separators are generally placed at the furnace exit and typically are not cooled. They are placed at the furnace outlet to protect the downstream heating surfaces, such as secondary and primary superheater surfaces, from erosion by solid particles. Thus, the U-beams are exposed to the high temperatures of the flowing stream of flue gas/solids, and the materials used for the U-beams must be sufficiently temperature resistant to provide adequate support and resistance to damage.
Impact type particle separators which are cooled or supported off a cooled structure are known. See, for example, U.S. Pat. No. 6,322,603 B1 to Walker, U.S. Pat. No. 6,500,221 B1 to Walker et al., and U.S. Pat. No. 6,454,824 B1 to Maryamchik et al.
A known impact type separator CFB boiler arrangement offered by The Babcock & Wilcox Company, based on an entirely water-cooled setting, is shown in FIGS. 1, 2 and 2A. This arrangement provides a furnace 10 having a gas-tight enclosure 11 suitable for operating with a positive pressure in the furnace 10, and provides a gas flow path for flue gas 15. It has no high temperature refractory lined flues in the vicinity of the primary particle separator U-beams 32 or in-furnace U-beams 34 and therefore requires minimal building space and reduces furnace refractory maintenance. This construction is possible due to the use of an impact type primary solids separator (U-beams 32) integrated into the boiler enclosure 11.
Fuel and sorbent are fed to the CFB bed through the lower front wall of furnace 10. The ash and spent sorbent are removed through drain pipes in the floor. The solids collected by the U-beams 32, 34 and multi-cyclone dust collector are returned through the rear wall to the lower portion of furnace 10.
Primary air enters furnace 10 through the distributor plate and secondary air is injected at elevations approximately 6 and 12 feet (1.8 and 3.7 m) above the distributor plate through upper and lower overfire air headers.
The primary solids separation system, generally designated 30, includes staggered rows of U-shaped channel members, or U-beams 32, suspended from the boiler roof. Material striking the U-beams 32 is separated from the flue gas 15, flows down the U-channel and discharges from the bottom.
A circulating fluidized bed (CFB) boiler furnace has substantial thermal inertia, which is attributed to hot bed material and un-cooled parts of the solids separator at the furnace exit such as U-beams, hot refractory, etc. In case of plant power loss, a.k.a. a black plant condition, the Main Steam Stop Valve (MSV) typically closes to prevent a rapid steam/water side pressure reduction and water level drop in the boiler. The thermal inertia of the drum, tubes, headers and other boiler components will continue to promote steam generation lasting after the MSV closing. In order to prevent steam pressure buildup that would trigger a safety valve opening with a corresponding rapid water level drop in the boiler, and to provide cooling of superheater surface subjected to residual heat of the un-cooled parts of the boiler components, such as a CFB boiler provided with U-beam solids separator, a steam relief valve would open allowing steam to bleed through the steam side of the superheater into the atmosphere or to the steam user (e.g., when the steam is used for heating), typically in a controlled manner.
As in the case of an open MSV or safety valve, this steam bleed results in a lowering of the water level in the boiler circulation system. If the water level recedes below the furnace roof, it will result in portions of the tubes being un-cooled, and those un-cooled tubes which are exposed to the residual heat of the un-cooled parts of the solids separator may be damaged. In order to prevent this from happening, the boiler may be provided with sufficient steam drum capacity and/or an independently powered boiler water pump that would maintain a safe water level in the boiler. However, providing this extra capacity of the steam drum and/or an independently powered boiler water pump adds to the boiler cost.
The combination of low temperatures and staged combustion allows fluidized-bed boilers, such as CFB boiler systems, to operate with low NOx emissions. Further NOx reduction can be controlled to lower values through the use of a selective non-catalytic reduction (SNCR) system consisting of ammonia injection near the U-beam elevation. An ammonia-based SNCR system includes storage and handling equipment for the ammonia, equipment for mixing the ammonia with a carrier (such as compressed air, steam or water) and injection equipment. The injection system, a key component, consists of nozzles generally located at various elevations on the furnace walls to match the expected flue gas operating temperature.
For additional details of the design and operation of circulating fluidized bed boilers and SNCR systems, the reader is referred to Chapter 17 and pages 34-13 to 34-15 of Steam/Its Generation and Use, 41st Edition, The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., © 2005.